Viruses are the largest group of pathogens, and there are viruses virtually everywhere. Nowadays, with the new techniques of genomic analysis, viruses are being identified in places where before there had been no previous success. It is estimated that the total number of viral particles on Earth is around 10 followed of 31 zeros, a number difficult to imagine, but that can be 100 million times more than the number of stars in the Universe (and infinitely superior to those of Hollywood). There are viruses in the oceans, primarily bacteriophages or bacterial viruses. These viruses lyse bacteria and play a key role in recycling nutrients and keeping the seas (and atmosphere) healthy. There are viruses in extreme environments, such as Antarctica or in water springs.
Viruses could be in space, though it seems unlikely. In animals, including humans, the concept of viroma is becoming increasingly consolidated. The viroma is the combination of all the viruses of the organism, including the non-pathogenic and those that produce acute or persistent infections, or even those that remain integrated into the genome of the animal, as we will see later. Each animal has a unique viroma in which the balance of the species forming it can change suddenly. The way of life, age, geographical location, season of the year, and many other factors can affect the exposure to virus, whose effect depends largely on the host and its immune status.
The number of different viruses that may constitute the viroma of mammals is around 320,000, of which we have only discovered a fraction. The majority of animal viruses are bacteriophages and mainly affect bacteria in the intestine and in other mucous epithelia, lysing them, releasing nutrients or causing other beneficial or harmful changes. The International Committee on Taxonomy of Viruses (or ICTV) is an agency which organizes virus into orders, families, subfamilies, genera and species. It classifies them according to the type of nucleic acid (DNA or RNA, single- or double-stranded, polarity, linear or circular, single or fragmented), the presence or absence of reverse transcriptase, the presence or absence of envelope, and the strategy for viral replication.
But within each species there is also great diversity, especially in viruses with RNA, which mutate very easily, sometimes giving rise to modifications of proteins enabling them to escape the immune response. Virologists analyse sequences of viral genomes and use computer programs to see the relationships between them, making what is known as phylogenetic analysis, which results in dendrograms. These studies are useful to determine the epidemiology of infections, (i.e. how they start, where they come from, how they evolve, etc.). The ease of travel that we have nowadays allows viruses to quickly travel over long distances. At the end of this activity, we recommend a link which shows the airplane activity throughout the day across the planet. You’ll be amazed!
This travel activity means that we now need global surveillance for most diseases. A virus that causes disease in Germany, for example, can reach Australia in a matter of hours and vice versa. Several international agencies oversee monitoring and control of both human and animal infections. Amongst the former, the best known is the World Health Organization (WHO), but there are many others as well. As for animal infections, the international agency for identification and control is called the World Organization for Animal Health, which is known by its original name of OIE or the International Office of Epizootics. Other similar entities are the FAO, which depends on the UN, and EMPRES-i, a global information system.
In this activity, we have learned how the huge amount of viruses that exist on our planet is organized, and that, in a specific host, it constitutes the virome. Due to the great “mobility” that viruses have acquired thanks to the ease of long journeys, various organizations have been established that help ensure that infections are controlled and not disseminated. In the following step we will see how, despite all measures, viral diseases are transmitted between hosts.